Proof of Earth`s Shape and Size
Transcript Proof of Earth`s Shape and Size
What shape best describes earth?
Describe the evidence that supports your
Oceans are flat
Airplanes all fly level on earth
Ancient Egyptians: The sky was a tent
canopy stretched between mountains at
the four corners of the Earth.
Inca: Called their land Tehuantinsuyu:
"The Four Corners of the Earth"
HOW DID THEY PROVE THAT
THE EARTH IS ROUND?
Ships appear to sink gradually
This is the star that
lies in space
practically over the
If you stood at the
North Pole, Polaris
Pole of the
Polaris – North Star
If you can spot Polaris in the sky, you can
always tell which way is north. Because of
this, Polaris was the most important star
for navigating at sea.
Why do observations of Polaris help
determine the Earth’s shape?????
The North Star appears lower and lower in
the sky as you travel toward the equator
because of earth’s spherical shape, where
the North Star is just visible at the
horizon. The latitude of the equator is 0°.
Polaris Changes in Altitude – Polaris
is a fixed point above the North
Polaris – Fixed Point above the
Because of this, in the Northern Hemisphere, the
altitude (angle measured in degrees above the
horizon) of Polaris tells observer his latitude
If observer’s latitude changes in the Northern
Hemisphere, the altitude of Polaris will exactly
match observer’s latitude.
ALT OF POLARIS = LAT OF OBSERVER
Locating Polaris – our latitude is
41°N , Polaris’ altitude will be 41°
Only a sphere can cast a shadow that
appears round. During a lunar eclipse, the
earth casts its shadow on the moon during
the full moon phase.
Summary: How did they prove
the Earth to be round?
Ships appear to sink GRADUALLY below the horizon as
they travel away from observer
Polaris (North Star) changes altitude (angle measured in
degrees above earth’s surface) directly with your latitude
ALT OF POLARIS = LAT OF OBSERVER
Lunar Eclipses - Earth’s shadow on the moon
Satellite Imagery – absolute proof
Is the Earth Perfectly round?
The Earth is NOT a perfect sphere
It is flattened at the poles and bulges at
Earth is slightly out of round or OBLATE.
What proof is there that we are
Gravity measurements. Gravity is the force of
attraction between any 2 objects.
Increase mass of objects = increase gravity
Decrease distances = increase gravity
If Earth were a perfect sphere, it would be
expected to exert an equal force on objects at
equal distances from the center of earth.
Weight!!! – measure of
We are further from the center of the
Earth at the equator – gravity is less
This means we weigh LESS!!!
We are closer at the poles…
We weigh MORE!!!!
The least amount of gravity is
farther from the center of Earth
High Mountains farther from the center of earth – gravity is
The Oblate Sphere
The difference is small
We can not see it with the naked eye
It still appears like a sphere
Be careful on multiple choice
Why is the earth not perfectly
Earth’s rotation causes…
Bulging at equator
Flattening at the poles
How big is Earth?
Having established the shape, what is the size?
How do you measure something really big?
Two centuries before the birth of Christ …
(-200) Eratosthenes devised a way to determine
the size of earth.
NOT CHRISTOPHER COLUMBUS!!!!!
Based his calculation on observations of
the sun on June 21, the Summer Solstice
in Alexandria and Syene – 2 cities in
On this date, the sun was straight
overhead at noon in Syene, but not quite
overhead at Alexandria
Obelisk in Alexandria, Egypt
The obelisk is called
and it was moved to
Central Park in 1881.
Made assumptions that…
a. The earth is round
b. The sun’s rays are parallel
Using simple geometry he set up an
equation that determined the
circumference of the earth.
240,000 stadia (about 39,250 km) is quite
close to modern measurements.
Calculate the percent deviation (Equations
are located on what page of ESRT?) if the
actual circumference is 40,070 kms.
Eratosthenes' work was lost, except for a
description of his method in an obscure source
140 AD: Ptolemy got an answer smaller by 28%
using a sound method with questionable data.
Both results known by King Ferdinand’s experts.
Columbus argued for the smaller value for the
distance from Spain to India but he was turned
down because they thought the distance too far.
In the end Queen Isabella overruled the experts,
and the rest is history.